In recent years, fiberoptic cables have replaced traditional copper wire as the preferred medium for telecommunications. Although optical fibers have certain advantages over copper wire, they are still subject to faults which may result during installation of the fibers or from environmental factors after installation. Thus, as the complexity of the telecommunication fiber network increases, the fiberoptic cables used within this network need to be constantly monitored to insure their performance and transmission quality.
One device which has established itself as a versatile instrument for monitoring the performance of fiberoptic cables is the optical time domain reflectometer, commonly referred to as an "OTDR." The time domain reflectometer makes use of the fact that microscopic fluctuations of the refractive index and small flaws in the optical fiber cause light to be reflected. In its simplest construction, an OTDR includes a light source, such as a pulse laser diode, for introducing a test pulse of light into the fiber under test (FUT), a photodetector for detecting the intensity of signals reflected back to the OTDR, and a timing device for measuring the elapsed times between introducing the test pulse and receiving the reflected signals.
When a test pulse of light is introduced into a FUT, the signal travels down the length of the fiber and runs into faults and imperfections along the way. This in turn causes the test pulse to be backscattered and reflected within the fiber core. These backscattered and reflected signals travel back down the FUT in the direction of the OTDR and are sensed by the photodetector. The trace signals of the backscattering and reflections provide clues as to the existence and location of faults within the FUT.
Backscattered signals (also known as Rayleigh scattering) are typically weak, and are due to refractive-index fluctuations and inhomogeneities in the fiber core. A backscattered signal may be used to detect faults such as micro-bends or splice losses, and to measure overall attenuation of light signals transmitted through the optical fiber. Reflective signals (also known as Fresnel reflections) are due to discontinuities in the fiber. A reflective signal may be used to determine the overall length of the fiber line, and to detect breaks in the fiber, reflective connectors, and splices of fiber having different indices of refractions. The operation and use of an OTDR to monitor the transmission quality of an optical fiber cable is generally known to those skilled in the art, and therefore need not be discussed further.
Most conventional techniques employing the use of an OTDR to monitor the transmission quality of an optical fiber cable utilize a technique known as "dark fiber monitoring." Such a technique is illustrated in FIG. 1 of the drawings of the present application. As can be seen in FIG. 1, an optical fiber cable includes a plurality of optical fibers, each of which can be used for an independent purpose. Most of the optical fibers within the cable are used to carry an optical signal from an optical transmitter to a respective receiver. The fibers which are used to carry this optical signal are called "active" fibers. The unused fibers within the cable are called "inactive" or "dark" fibers. The technique of dark fiber monitoring consists of optically connecting an OTDR to one end of an inactive or dark fiber which is part of the optical fiber cable to be monitored. Once the OTDR is connected to the dark fiber, a short test pulse light is introduced into the optical fiber by the OTDR, and travels the length of the fiber. When the test pulse light reaches a fault within the fiber, part of the test pulse is reflected back to the OTDR. When the OTDR detects this reflected signal, it records the intensity and time of arrival of the signal. The OTDR repeats this process for every reflected signal which is detected. A comparison of the relative intensity of the reflected signals is then used to determine the transmission quality of the cable. A reflected signal of relatively high intensity is typically generated from a fault within the fiber, whereas a reflected signal of relatively low intensity is typically generated from Rayleigh scattering.
Once it has been determined that a fault exists within the optical fiber, its location can be precisely determined by the equation F=(c/2n)t, where c is the velocity of light, n is the maximum value of the refractive-index of the fiber core, and t is the elapsed time measured from the time of departure of the initial test pulse to the time of arrival of the reflected signal.
Although dark fiber monitoring is useful for detecting the existence and location faults within an optical fiber cable, the technique itself has a number of limitations. By far one of the biggest limitations of dark fiber monitoring is that the technique can only be used to detect faults located within the unused optical fiber of the fiber cable which is being tested; the technique cannot be used to detect faults which exist solely within one or more of the active fibers within the optical fiber cable. Thus, when the entire optical fiber cable has been damaged, such as at a particular location, dark fiber monitoring is a useful technique to pinpoint the location of the damaged part of the fiber cable. However, if only part of the optical fiber cable is damaged, whereby faults are created only within a portion of the active fibers and not in the dark fibers of the fiber cable, dark fiber monitoring will not be useful for determining the location of the faults within the damaged active fibers. This is because the technique of dark fiber monitoring does not monitor the transmission quality of an active optical fiber; it only monitors the transmission quality of the unused or dark optical fibers within the fiber cable.
Because of the limitations associated with dark fiber monitoring, new techniques have been developed wherein the transmission quality of the active optical fiber itself is monitored. This technique is referred to as "active fiber monitoring." The basic principle behind active fiber monitoring is similar to that of dark fiber monitoring with the exception that the fault locating device is optically coupled directly to an active optical fiber using an optical coupling device such as a wavelength division multiplexer (WDM). However, as with dark fiber monitoring, there are several problems associated with conventional techniques of active fiber monitoring. First, any test light pulse introduced by an OTDR into an active optical fiber will destablize the respective optical transmitter of that optical system. This occurs as a result of either the test pulse or reflections of the test pulse entering the optical transmitter. Second, light signals sent by the optical transmitter through the FUT will be detected by the OTDR and thereby degrade the accuracy of the OTDR measurement. Third, some of the reflected signals of the OTDR's test pulse will be received by the respective optical receiver of that optical system, thereby increasing the bit error rate of that receiver.
Thus, the monitoring of the transmission quality of optical fibers using dark fiber monitoring and active fiber monitoring have not proved to be as effective as desired. It is therefore an objective of the present invention to provide a technique for actively monitoring the transmission quality of an optical fiber which overcomes the aforementioned limitations.